Amen brother. Dual exhausts cost power on 4 cylinders, you need the flow down a proper sized pipe to scavenge head better. Generally more noise, hey you bought the four, ALL fours have potential to make more noise, the exhaust pulses are far enough away from each other they do not interfere with each other to kill noise. Look how single exhaust on a V-8 kills noise compared to duals, the cramming in there of all those sound waves kills power but kills noise too.
You can forget any ideas of necessary back pressure as old wives tales that cost you power. You want pressure as low as possible.